Washing machine overflow

[nikkoD]

The overflow in washing machine seems to overflow when machine is draining.
I can hear water draining away however as the machine pumps more water into the pipe it seems to be unable to cope and ends up overflowing.
This has only happened recently and have had a plumber and dynorod out to put acid and metal rod thing down the pipes. Pipes seem to be clear of any blockage. Both contractors were stuck as to what the problem may be.
I'm in a 2nd floor flat.
I'm stuck with what to do next

 

[dal5band]

Is there a trap (U bend) immediately below where the W/M empties into the pipe. Check this trap - it may be partially blocked with soap or a sock.